Florence: Oh, look at that mountain of dirty dishes! Who is going to wash
all of those?
Jack: I promise I will do them when I get home from work.

Why "going to" and not "will "is it because there are evidences that the dishes is going to be done or is it because it has been planned before. I think the first solution is right, there are a mountain of dirty dishes so I can conclude that the dishes is going to be done soon
Am I right?
//www.englishpage.com/verbpage/verbs23.htm

The "generic" reason for using is going to rather than will is because the "Continuous Future" form emphasises connection to the present (right now, time of utterance). But I don't think many native speakers would recognise because it has been planned before as any kind of "intended implication" in your exact context. I'd say the continuous form is in fact more likely in your cited example - but that's simply because the speaker is interested in establishing right now who will do the washing up (regardless of whether it'll actually get done immediately, or perhaps much later)
– FumbleFingers Reinstate MonicaJun 12 '18 at 14:32

2

That's one way to describe one possible reason for using the construction, yes. But it's not applicable to your specific context. More relevant might be We also use is going to when we're talking about something that's just about to happen. For which a clear-cut example might be I will be sick (possibly, if I eat that; not necessarily right now), which can be contrasted with I am going to be sick (very "immediate" - I'm likely to vomit as soon as I finish speaking).
– FumbleFingers Reinstate MonicaJun 12 '18 at 15:46

2

(But you should bear in mind that native speakers wouldn't particularly distinguish between the two verb forms in your exact context; to all intents and purposes they're just two ways of saying the same thing.)
– FumbleFingers Reinstate MonicaJun 12 '18 at 15:49

2

@FumbleFingers just curious - why don't you generally put your thoughts into answers? You answer a lot of questions very well but don't even try to get points or badges for your answers. You're not the only one in this particular neighborhood of StackExchange who does the same thing, either. Seems like a cultural aspect of this site.
– dwilliJun 15 '18 at 20:10

1

@FumbleFingers I agree with dwilli. Answering-in-comments discourages others from posting an Answer which provides the same explanation, because it looks like they copied from you. So the Question gets the status of Unanswered. This wastes the time of those looking for Questions to answer, and confuses those looking for Answers.
– sammy gerbilJul 16 '18 at 20:43